This invention relates to a charging device to be employed in electrostatic photography.
As is well known, it is a common practice in electrostatic photography that a photoconductive insulating layer provided on a somewhat electrically conductive substrate is uniformly charged to form a "photosensitive layer", the photosensitive layer is thereafter exposed to light to form a latent electrostatic image thereon, and the latent image is developed employing electroscopic powders or "toners". In the above mentioned charging process, it is required that the substrate be connected to ground. However, it has been found heretofore that a substrate having a good flexibility is rather difficult to ground at a low resistance, and for this reason, it is difficult to charge the photoconductive layer uniformly. This is especially so when the substrate is made of polyester or triacetate resin film or other resin film, paper or resin coated paper, all of which are difficult to make conductive in the direction of their thickness.
Although it is a common practice that a photoconductive layer supported on such a substrate is electrically charged with its peripheral portion being grounded, the central portion of the photoconductive layer tends to be insufficiently charged because of the comparatively high grounding resistance.
In order to compensate the above described phenomenon and to obtain uniform distribution of the electrical charges, some measure must be taken to collect a further amount of electrical charges to the central portion of the photoconductive layer.
Furthermore, in electrostatic photography, there exists a problem of "edge effect" which causes thick peripheral portions and thin central portions in the image thereby developed. To compensate this "edge effect", it is required to collect more charges on the central portion of the image so that a uniform density is obtained throughout the image. In the case where a negative developing method is employed, an image of uniform density may also be obtained by reducing the amount of the electric charges at the central portion.
Therefore, a principal object of the present invention is to provide a charging device capable of collecting more electric charges on some part of a photosensitive surface than on another part thereof, or to charge the surface uniformly.
A method of charging a sheet-formed material is known, wherein an electrically conductive wire of a diameter ranging from 20 to 100 microns is employed as a corona discharging electrode (hereinafter called a corona wire), a high voltage is applied to the corona wire to cause corona discharge therearound, and the sheet-formed material is passed adjacent the corona wire to be charged with electrical charges.
Another method is also known wherein a conductive wire of a larger diameter than the corona wire is placed near the corona wire, and uniform distribution of electrical charges are accumulated on the sheet material.
All of these methods, however, were proposed for the purpose of obtaining a more uniform corona discharge.
According to the present invention, there is provided a corona charging device wherein a wire having a larger diameter than that of the corona wire and made either of an electric conductor, insulator, or insulator coated conductor (hereinafter called a discharge control line) is provided near the corona wire, and the discharge control line is disposed not in parallel to the corona wire but is varied in its distance from the corona wire, whereby the intensity of the corona discharge is partly varied, and the distribution of the electrical charges on the photosensitive layer is thereby controlled.
The nature, principle, and utility of the present invention will be more clearly understood from the following detailed description when read in conjunction with the accompanying drawing.